Probing the Onset of Dust Coagulation in Dense Clouds

Details Probing the Onset of Dust Coagulation in Dense Clouds Probing the Onset of Dust Coagulation in Dense Clouds

Mar 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop50363c&link_type=abstract

Spitzer Proposal ID #50363

Physics

Scientific paper

Our previous Spitzer IRS observations have confirmed the striking break-down of the diffuse ISM linear correlation of the depth of the 9.7 mic silicate absorption feature (tau(9.7)) with near-IR color excess (E(J-Ks)) at high extinctions in dense clouds. In addition, the observed shape of the 9.7 mic band changes between dense clouds and the diffuse ISM. Theoretically, these variations have been attributed to the effect of grain growth by coagulation on the extinction properties of interstellar dust, but the conditions responsible for grain coagulation in dense clouds is not well understood. The new observations we propose here in combination with already existing deep IR extinction maps, provide us with a new and unique opportunity to directly probe the onset of dust coagulation and the change in the physical properties of the dust in the environment of a single nearby dense cloud: the Pipe Nebula. The goals of our proposal are: - We will measure the change in tau(9.7) as a function of near-IR extinction and density in dense cloud cores and the surrounding lower-density intercore material. - We will analyze the 9.7 mic profile shape, to observationally determine under what conditions deviations from the diffuse ISM feature become apparent. - We will establish, observationally, the interrelationship between the flattening of the tau(9.7)/E(J-Ks) relation and the change in shape of the silicate profile. Our lines of sight span a wide range of accurately determined (IR) extinctions (Av ~ 5 to 48 mag). We will accomplish our above goals by obtaining high S/N IRS observations of a well-chosen sample of background stars behind the Pipe Nebula and two of its most opaque dense cores. The source list ensures that the extinction range is well-sampled spatially and in terms of extinction value inside the cloud cores and the surrounding intercore material. In this way, we will be able to observe changes in the dust properties as they occur within a single dense cloud.

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